Conventionally, retroreflective sheeting capable of reflecting incident light back toward the light source has been well known, and such sheeting is widely used in the above-described fields of application owing to good visibility at night on the basis of its retroreflectivity. For example, road signs, construction signs and other signs using retroreflective sheeting have excellent characteristics in that, at night or the like, they reflect light from a light source (e.g., the headlamps of a moving vehicle such as automobile) back toward the light source (i.e., the moving vehicle) and afford good visibility to the viewer of the signs (i.e., the driver of the vehicle), thus permitting the transmission of correct information.
Thus, retroreflective sheeting generally reflects light from a light source back toward the light source and hence affords good visibility in the direction of the light source. However, when the viewer is positioned in a direction different from that of the light source, its visibility is markedly reduced. Moreover, owing to the nature of retroreflective sheeting, the difference between the angle of incidence of light from the light source of a vehicle such as an automobile and the angle of observation of the driver of the vehicle increases as the light source comes nearer, resulting in a marked reduction in visibility. Recently, with the development of road systems and the diversification of provided information, the amount of information displayed on each sign is increasing. With consideration for the speed of vehicles, it may be very difficult for the drivers to read necessary information in a very short period of time when they are within the visible range of retroreflected light.
Accordingly; in the case of applications requiring the provision of more correct information, pronounced propaganda effects and the like, conventional retroreflective sheeting having retroreflectivity alone is limited in visibility. In particular, there is a strong demand for excellent retroreflective sheeting which can always afford good visibility to viewers at night even when the viewers are positioned in directions different from that of the light source.
In order to meet this demand, various attempts have been made to improve the visibility of retroreflective sheeting. For example, Japanese Patent Laid-Open No. 173008/'93 discloses encapsulated lens retroreflective sheeting having both retroreflectivity and light-storing luminosity wherein a transparent resin layer is used as a support for partially embedding retroreflective microspherical lenses (i.e., microspherical lenses having a deposited metal film coating the approximately hemispherical surfaces thereof and hence exhibiting retroreflectivity) and a layer of a light-storing luminescent substance is disposed on the back surface (i.e., the surface opposite to the light incidence side) of the support. However, owing to the structure of this retroreflective sheeting, the light radiating from the luminescent substance is intercepted by the embedded retroreflective microspherical lenses, so that the amount of light emitted thereby is very small. Consequently, this retroreflective sheeting is totally unsatisfactory for the purpose of improving visibility.
Moreover, the specification of PCT International Application Publication No. WO93/14422 discloses photoluminescent encapsulated cube-corner retroreflective sheeting which contains a phosphorescent pigment in the bonds for bonding the cube corner-forming surface to the support, and also suggests a method for imparting fluorescent properties to encapsulated lens retroreflective sheeting. However, even in the retroreflective sheeting described in the specification of this international application, the phosphorescent pigment fails to emit a sufficient amount of light. Consequently, signs made of this retroreflective sheeting will not have such a high degree of visibility that the information displayed thereby can be recognized from some distance.
Furthermore, the specification of PCT International Application Publication No. WO96/18920 discloses ultraviolet-excited luminous retroreflective sheeting of the encapsulated lens structure wherein the bonds at which a base sheet having retroreflective microspherical lenses partially embedded in a support is partly bonded to a protective film are formed on the protective film or base sheet as an ultraviolet-excited luminous layer by printing an ultraviolet-excited luminous resin composition, and these bonds are then fused to the base sheet or protective film by the application of heat or bonded thereto with the aid of a suitable adhesive. Moreover, this specification also suggests ultraviolet-excited luminous retroreflective sheeting wherein an ultraviolet-excited luminous support is prepared by forming the support of an ultraviolet-excited luminous resin composition or providing at least on the microspherical lens-embedding side of the support with an ultraviolet-excited luminous layer formed of an ultraviolet-excited luminous resin composition, and this support is partly heated and melt-formed by means of an embossing roll or the like to form bonds for bonding the protective film partly to the base sheet.
However, the retroreflective sheeting described in the specification of the aforementioned WO96/18920 involves problems in that the ultraviolet-excited luminous layer consists essentially of only the bonds for bonding the protective film to the base sheet and the proportion thereof to the total surface area of the retroreflective sheeting is very small and in that the amount of luminescent substance incorporated in the bonds cannot be increased so much. Consequently, it has been found that the amount of light emitted by the luminescent substance tends to be insufficient and, moreover, the retroreflective sheeting also has disadvantages such as poor adhesion of the ultraviolet-excited luminous layer to the protective film and difficulty in forming an adhesive layer on the formed ultraviolet-excited luminous layer.
On the other hand, several methods for embedding retroreflective microspherical lenses in some part of a support are also known. For example, U.S. Pat. No. 4,075,049 discloses a method for the preparation of a base sheet which comprises the steps of embedding the approximately hemispherical parts of glass beads in a temporary support (e.g., polyethylene-laminated paper) so as to form a mono-layer, depositing a lightreflecting material on the glass bead-bearing surface thereof, forcing part of the glass bead layer into the polyethylene layer of the temporary support by heating and pressing the glass bead-bearing side of the temporary support by means of a mold having a raised pattern, and pressing a support having an adhesive layer against the glass bead-bearing side of the temporary support to transfer the glass beads not buried in the polyethylene layer to the support.
However, the purpose of the method described in this U.S. patent lies in the fact that, in the fabrication of encapsulated lens retroreflective sheeting, the support of the base sheet is thermoformed by means of a mold having the same raised pattern as the raised pattern of the aforesaid mold so as not to incorporate glass beads in the bonds for bonding a protective film to the base sheet. Although this method may be employed in cases where it is desired to prevent a very small part of the glass bead layer from being transferred to the support, it is not easy to prevent a large part (e.g., more than half) of the glass bead layer from being transferred to the support and, therefore, this proposed method is not considered to be suitable in such cases.
It is an object of the present invention to provide lens type functional retroreflective sheeting which have both retroreflective regions and visual functionality presenting regions and hence exhibit good visibility even at night or the like, as well as methods of producing the same.